Turbine blades are usually unitary structures with an airfoil portion, a platform portion and a root portion. Turbine blades are typically formed by casting, which may additionally involve the use of directional solidification techniques. The casting yield of turbine blades, especially those made of advanced single crystal alloys, can be as low as about five percent. One of the primary reasons for such low yields is casting defects. Examples of casting defects include secondary nucleated grains, freckle chains or other grain boundaries.
Such defects commonly occur in areas where there are large geometric changes in section size, particularly where there is a large increase in section size. In a turbine blade, one such area is in the transition region between the airfoil and the platform. Defects are frequently initiated in this transition region when the blade is formed using directional solidification techniques with the airfoil portion down and the root portion up, so that the direction of crystal growth is from the blade tip toward the root. In such case, there is a large increase in cross-sectional area as the airfoil portion transitions into the platform portion. Another area where defects have been known to develop is in the root portion of the blade, such as on the root shank.
Thus, there is a need for a turbine blade construction that can minimize the formation of such casting defects.